In a typical clutch assembly for an automatic transmission (as illustrated in FIG. 2), a clutch pack 16 is provided for providing a frictional engagement between a first member 12 and a second member 14 which is rotatable relative to the first member 12. A piston 60 is provided within a fluid chamber 62 such that the application of fluid pressure to the fluid chamber 62 causes the piston 60 to move into engagement with the clutch pack 16 to frictionally engage the second member 14 to the first member 12. The contacting of the friction plates with increasing pressure eventually causes the rotation of the second rotatable member 14 which the system is designed to engage.
A problem that has been recognized in the art for a typical clutch assembly of this type is that the centrifugal force of the fluid within the fluid chamber 62 can put a positive pressure on the apply piston 60 as illustrated in FIG. 2. The pressure generated by the centrifugal force of the fluid can cause unintended engagement of the clutch pack 16. Thus, a balance piston system has been developed as illustrated in FIG. 3 in which a balance piston 70 is disposed between the apply piston 60 and the clutch pack 16 so that hydraulic fluid is present on opposite sides of the apply piston 60. When the balance piston system is spinning, the balance piston 70 traps fluid at the outer edge of the cavity 63 that it creates with the apply piston 60 to counteract the centrifugal forces caused by the fluid on the other side of the apply piston 60 as illustrated in FIG. 3. The balance piston 70 is provided with a seal member 72 that contacts an inner surface of the axially extending arm 64 of the apply piston 60. The seal 72 is critical for the proper functioning of the balance piston system. The apply piston 60 includes a first inner diameter seal 74 and second and third outer diameter seals 76, 78, respectively. Each of the seals 74, 76, 78 are loose seals which are received in a respective recessed groove 80a–c formed in the surface of the apply piston 60. In order to form these grooves 80a–c, the apply piston is formed in a casting process in which the grooves 80a–c can be cast or machined. The seals 74 and 76 combine to seal an apply chamber for the apply piston 60 while the seals 76 and 78 each engage an outer diameter surface of the piston chamber 62 along opposite sides of a fluid flow path 82 provided in the outer diameter surface of the apply piston 60. Fluid flow path 82 allows fluid to pass from piston cavity or chamber 63 to another piston chamber (not shown).
Although the prior art piston and seal design is adequate for its intended purpose, it is desirable to provide a piston and seal design that is easier to manufacture and to assemble into a clutch system.